Television system



Dec. 22, 1942.

A Haar/WEH- F/L '/'ER l. #MPL/F7171? l Low P5166 WL 'ffm ou'rPL/'r NEG. POL/mmf T. L. GOTTIER TELEVISION SYSTEM Filed Dec. 31, 1940 2 Sheets-Sheet l Zhwentor t Le r Cttorncg Dec. 22, 1942. T. l.. GOTTIER TELEVISION SYSTEM 2 Sheets-Sheet 2 Filed DeG. 31, 1940 FREQUENCY of' CRYG vwz. OSC. Z

/4 FREQUENCY (Gu/WD FWE?) @Hex-neu l an improved frequency modulation system for a Patented Dec. 22, 1942 A UNITED STATES PATENT oFF-cE TELEvrsIoN SYSTEM Thomas L. Gottier, Westmont, N. J., assignor to Radio' Corporation of America, a corporation of Delaware Application December 31, 1940, Serial No. l372,470A

modulating being referred to as negative and positive modulation, respectively. The modulating signal referred to is a mixture of synchronizing pulses and picture signal, the said pulses being in the direction of black" in the picture. 1

An object of the present invention is to provide an `improved frequency modulation system for a television or picture transmitter which retains the above-mentioned feature of having the synchronizng pulses always go to a predetermined modulation level. lThis predetermined level in a frequency modulation system is, of course, a predetermined frequency. y A further object of the invention is to provide television or picture transmitter in which the plcture signals in the white direction, or at least 14 claims. (ci. 17s-7.2)

If desired, the frequency swing of the carrier wave oscillator may be prevented from swinging beyond a fixedfrequen'cy limit in the direction of white-in the picture by employing a Amethod similar to that just described but differing in that the controlling voltage supplied from the discrlminator network is applied to an ampliiler in certain frequency components thereof, cannotvexceed a predetermined modulation level.

A further object of my invention isto provide an improved method of and means for hold.. ing the frequency deviation or frequency swing of a frequency-modulated carrier wave within lpredetermined frequency limits.

` A still further object of the invention is to limit that frequency deviation ofa frequency-modulated carrier wave which l-s produced bycertain frequency components', such as low frequency components, of the modulating signal.

In one preferred embodiment of the invention, the peaks of the synchronizing pulses are held at the desired predetermined level 'or frequency by utilizing a stable oscillator, such as acrystal oscillator, to .provide voltage at a guard frequency, and by beating-this voltage with the output of the carrier wave oscillator. The resulting beat frequency voltage'is fed to a frequency discriminating network such as a low pass filter having a cut-off frequency close to the beat frequency. If the beat frequency changes, the filter r output changes. and this change is utilized to shift thel carrier wave oscillator frequency back to a value where the peaks. of the synchronizing pulses swing the oscillator to the desired predetermined r.

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the modulating signal channel. In the example under consideration, it is so applied to a video amplifier that any increase in lthe amplitude of the low frequency components of the picture sig' nal in the white direction which swings the oscillator beyond the frequency limit will lower the A gain of this amplifier.

The invention will be better understood from the following description taken in connection with the accompanying drawings in which Figure l is a circuit diagram of a portion of a television transmitter'embodying my invention,

'Figure 2 is a diagram which is referred to in explaining the operation of .the invention,

Figures 3 and 4 are curves showing the pass characteristics of certain filters shown in Figure 1.and

Figure 5 shows oneform of frequency discriminator that may be used to replace the low pass filters of Figure 1.

Referring to Figure 1, there is shown a television transmitter comprising a carrier wave osv cillator Ill which may be of a well-known type having a tuned circuit II, one end of which is coupled' to the .grid ofthe oscillator tube I3 by acondenser I2. v v.

The output of oscillator I0 is frequency-modulated by the video or television signal input which is amplifled by a video amplier I6 and applied through a blocking condenserY I1 and an inductance coil I8 to the grid of. a reactance tube I9.-

The function of the reactance tube I9 is to vary the effective reactance across the oscillator-tuned circuit II in accordance' with the voltage amplitude variations on the'grid of the reactance tube. Thus, a frequency-modulatedr` carrier is obtained. In the particular example shown, the tube I9 is -connected across the tuned circuit 'II by a connection from the plate of tube I9 tothe plate of oscillator tube I3 and by a connection from the cathode of tube I9 to ground. A resistor 2| connected between the grid and plate of tube I9 through a blocking condenser 22 functions, to-

gether with the grid-cathode 'capacity indicated at 23, to provide the necessary out-of-phase componentfor the reactance tube operation.

a diode ze is provided to remsm ,the einen .current component of the video signal. The D. C.

reinsertioncircuit operation is well known and may be explained briefly as follows: The synchronizing pulses are of positive polarity at the plate of diode 26 and will cause a fiow of diode current whenever they drive the diode plate posi. tive with respect to its cathode. This puts a charge on coupling condenser I1. At the end of a pulse the condenser I1 loses a small part of its charge through a path which includes a high lmpedance resistor 21 whereby the '1).A C. voltage across condenser I1 may always have a value determined by the peaks of the synchronizing pulses as measured from the A. C. axis of the signal.

The battery 26 applies a negative bias to the control grid of tube I9 so that the synchronizing pulses plus the radio frequency voltage never drive the grid of tube I9 positive with respect to its cathode. v

Inductance coil I8 is provided to separate/the electrode capacities of tubes I9 and 26 for the purpose of holding up the high frequency response. Coil I8 also functions to prevent the oscillator from feeding back radio frequency energy.

Referring now to my method and circuit for holding the peaks of lsynchronizing pulses at a fixed frequency, a stable oscillator, such as a crystal oscillator I, is provided which produces a voltage at a frequency slightly beyond the said .fixed frequency representing peaks of synchronizing pulses.

Referring to Figure 2, the frequency of oscillator l is represented as f5 and may be referred to as a guard frequency. The frequency at which the peaks of lsynchronizing pulses are to be held is represented as f4. The synchronizing pulses and the picture signal which comprise the modulating video signal are indicated at 3| and 32, respectively.

The output of oscillator I is fed to the grid of a mixer tube 36. Also, signal from the carrier wave oscillator I is fed to another grid of mixer` tube 36 through amplifier tubes 35 and .31. Amplifier 35 may have its plate circuit tuned to the frequency f4 representing peaks of synchronizing pulses. The voltages at frequencies f4 and f5 beat in the mixer tube 36 to produce a voltage at the difference frequency f5-f4 which is supplied to a frequency discriminating circuit such as alow pass filter IA.

The filter IA may have the characteristic shown in Fig. 3, the beat frequency f5-f4 being located slightly beyond the filter cut-ofi' frequency, as indicated in Fig. 3, when the peaks of pulses 3| are at the correct frequency f4. If the beat frequency changes, the voltage output of lter IA changes. This output is amplified in an amplifier 40 and supplied to a rectifier-filter unit 4|. By rectifying and filtering the ampliiied output in the unit-4|, there is obtained a biasing voltage which is supplied to the grid of reactance tube I9 through a low impedance resistor and through resistor 21 and coil I8, this biasing voltage changing in accordance with any change in the filter output. In the example shown, the rectifier 4I supplies a negativebias. A low impedance resistor 34 connected between the diode 26 and battery 28 provides-the necessary impedance between ground and the point at which it is applied. If the frequency representing the peaks of pulses 3| (Fig. 2) increases to a frequency higher than f4, there is a decrease in the beat frequency fed to filter IA with the result .that there is an increase inthe negative bias on the reactance tube I9 which lowers the frequency of oscillator I0, bringing it back to the desired value f4 during the said peaks.

Preferably, the above-described circuit is operated in one of two Ways to insure that the frequency which beats with the guard frequency f5 is the one representing the peaks of pulses 3|. One way is to key the mixer tube 36 by means of positive pulses supplied from a suitable source 42 so that a beat frequency is produced in the mixer only during the occurrence of the synchronizing pulses 3l. When operated in this way the mixer 36 is biased to cut-ofi in the absence of keying pulses by means of a biasing source 43. An inductance coil 44 and a resistor 45 are provided merely to provide impedance in the grid circuit of the mixer tube so that voltages from units I and 42 will appear on the control grid of the mixer tube 36. v

Obviously the keying pulses preferably should occur during and only during the occurrence of the synchronizing pulses 3|. For this reason it may be desirable to make the keying pulses narrower than the pulses 3|.

When the circuit is operated with keying pulses, the amplifier tube 31 may be biased to operate as a class A amplifier or it may be biased t0 pass only the larger amplitude frequency components. Also, the amplifier 35 may be untuned if preferred.

The second way of operating the circuit is to omit the keying pulses, lower the negative bias from source 43 so that the mixer 36 will at all times mix any signals applied to its grids. and, finally, to bias tube 31 so that only the frequency components of high amplitude will pass through it and mix with the crystal oscillator output. The bias on tube 31 usually should be suflicient to bias the tube beyond cut-off.

For the case where it is desired to hold the peaks of pulses 3| at f4 and not merely to keep them from exceedingthe bias on the reactance tube I9 should besuch that, if the deviation control circuit were made ineffective, these peaks would deviate the carrier frequency'to a frequency slightly higher than f4. With this adjustment the control circuit is eective to prevent a frequency deviation in either direction.

It may be desirable to provide a guard frequency such Aas the frequency fl indicated in Fig.,2 to prevent the picture signal in the white direction from shifting the oscillator frequency too far on the low frequency side. Assume, for example, that it is desired to prevent the frequency deviation of the carrier wave oscillator I0 from going beyond f2. This isr accomplished by means of a circuit'similar to the control circuit previously described and includes a stable oscillator such as a crystal oscillator 2feeding intoy a mixer tube 5|. The frequency modulated output of oscillator I0 also feeds into the mixer tube 6I through amplifier tubes and 52. The amplifier 56 preferably is tuned to the frequency f2. The tube 52 is biased to pass only the higher amplitude signal components.

I'he output of mixer tube 5I is supplied to a frequency discriminating circuit such as a low rggss Filter 2A having 'the characteristic shown in The output of filter 2A is amplified by an amplifier rectified in a suitable rectifier-filter circuit 53 to produce a negative biasing voltage whiclhs is applied to the grid of the video amplifier So long as the frequency deviation of oscillator I0 does not go beyond f2, the lowest beat fre- Y quency produced in the mixer tube 5l is equal to ,f2-fl. If the deviation goes beyond this point, the beat frequency is lower than before, more signal passes through filter 2A and there is an increase in the negative bias applied to the Vgrid of video amplifier. It. This lowers the gain of amy plifier I6 whereby the frequency swing of oscillator l0 in thedirection of white in the picture is reduced.

lit will be understood, however, that'when both of the above-described control circuits are used at the same time the reduction in video amplifier gain does not cause the peaks-of the synchronizing pulses 3i to go to a frequency lower than f.

They vare always held at the modulationlevel ft by the control bias applied to the reactance tube Id Perhaps it should be noted with respect to the level setting of the synchronizing pulse peaks frequency. 1

It will also be understnod from the foregoing that my invention may be applied to a frequency modulation system by .employing a guard frequency on either side of the frequency deviation On the whi side of the modulated carrier there is no regularly recurring signal such as the synchronizing pulses on the black side. .However, a picture ordinarily contains some strong low frequency components as a result of the difference in shading (i. e. dark gray, light gray, etc.) of comparatively large areas in a picture. For these components the modulation index is high venough so that the amplitude of a side band frequency component at or near the limit frequency f2 is` greater than that of other side band components whereby the tube 52 (Fig. l) maybe biased to pass this one component to the exclusion of adjacent frequency components. Therefore, components having frequencies close enough to the frequency f2 to cause additional beat frequencies that might be pased'by the filter 2A are prevented from passing into themixer tube 5i. It is assumed, of course, that it is one of the above-mentioned strong low frequency components that swings the carrier wave oscillator to the limit frequency f2.

In. some cases it may be desirablevto peak the high frequency 'components of the transmitted signal. This may be desirable for the purpose of increasing the signal-to-noise ratio in a regionwhere alarge amount of noise is present.

This may be accomplished -by peaking the high frequency components of the modulating signal before it is applied to the modulator tube. 'As a result, the high frequency components will swing of the carrier wave without using a guard frequency on the other side, or by employing both guard frequencies at the same time, as illus-l trated.

It should be mentioned that there may be ceri tain limitations in the effectiveness fof the guard frequency fi used on the white side of the picture signaL These'limitations may also apply to a more or less extent to the application of one or more guard frequencies to a carrier wave which is frequency modulated by audio signals such as speech. It is Well known that a frequency modulated carrier wave when analyzed is found to have sine wave components consisting o'ffsignal components at a carrierfrequency and at a Vplurality of side band frequencies. Both the amplitude of the side band components and the width of the-side 'bands depends upon -the ratio of the absolute frequency deviation to the imposed modulating frequency,this ratio being known as the frequency modulation index?l This. is described in the July 1930 Proceedings of the I. R. E. in the article omFrequency modulation, by van der Pol, pages 1194 to 1205.

If the modulation index is large, the side band frequency is of large amplitude andsignal at this frequency may be passed through a tube biased beyond cut-0E to the exclusion of other side band components which have been produced by higher frequency modulating components.

Thus, on the synchronizing pulse side of the fre- 4quency modulated carrier wave, the side band component which is produced by the fundamental component of the line frequency pulse is of large amplitude compared with the side band components produced by the picture signal itself. In one embodiment of the invention the frequency deviation is 0.75 megacycle each side of the carrier frequency and the fundamental frequency of 'the line pulses is 13,230 per second, giving a modulation index of about 60. IBecause of this,

the carrier wave oscillator beyond the guard frequencies since the guard frequency control circuits will not respond to the high frequency deviations. These control circuits will continue to function as previously described, however.

'It will be apparent that my invention is not limited to the use of the specific circuits and apparatus previously described, For example, the low pass filters may be replaced by suitable resistbr-capacitor networks having the desired frequency characteristic. Such a network is illustrated in Fig. 5.

Also, the use of an oscillator to produce `a beat frequency is not essential although -it is used in the preferred embodiment of the invention. By properly designing the frequency discriminator IA, for -example, voltage at the frequency f4 itself maybesupplied thereto 'with frequency f4 located on the slope of the frequency-response curve. In' this case the slopeA of the discriminator characteristic curve should be. reversed since an increase 'in the deviation frequency beyond f4 now is an increase, rather than a decrease in the control frequency. A' high pass filter will give this reversed slope.

For filter design reasons orlthe like, it may be desirableto lower the frequency f4 at a point in or preceding the control circuit, as by frequency division, in an embodiment where the stable guard frequency oscillatorA is not used.l

I claim as my invention: r

l. A picture transmitter comprising a carrier wave oscillator, means for frequency modulating said oscillator in accordance with a signal comprising picture signals and recurring synchronizing pulses whereby the peaks of said pulses are represented by a certain maximum Vdeviation of said oscillator frequency, means for deriving a frequency control bias which changes in response to a change in the frequency representative of said peaks, and means for controlling the frequency of said oscillator in accordance with said control bias.

wave oscillator, means for frequency modulating said oscillator in' accordance with a signal comprising picture signals and recurring synchronizing pulses whereby the peaks of said pulses are represented by a certain maximum deviation of said oscillator frequency, means for deriving a frequency control bias which changes in response to a change in the frequency representative of said peaks, and means for applying said control bias to the frequency modulating means.

3. A picture transmitter comprising a carrier wave oscillator, means including a reactance tube for frequency modulating said oscillator in accordance with a signal comprising picture signals and recurring synchronizing pulses whereby the peaks of said pulses are represented by a certain maximum deviation of said oscillator frequency, and means for causing the peaks of said pulses always to swing the output of said oscillator to a predetermined frequency, means for producing a frequency control bias which changes in response to a change in the frequency representative of said peaks, and means for applying said control bias to said reactance tube.

4. A picture transmitter comprising a carrier wave oscillator, means for frequency modulating said oscillator in accordance with a signal comprising picture signals and recurring synchronizing pulses which go to a fixed voltage level whereby the peaks of said pulses are represented by a certain maximum deviation of said oscillator frequency, means for causing the peaks of said pulses always to swing the output of said oscillator to a predetermined frequency regardless of the tendency of said oscillator to drift in frequency, said last means including means for deriving a frequency control bias which changes in response to a change in the frequency representative of said peaks, and means for controlling the frequency of said oscillator in accordance with said control bias.

5. The invention according to claim 4 wherein said last means comprises means for producing a guard frequency and for beating said guard vfrequency with said predetermined frequency.

6. In a picture transmitter having an oscillator, the method which comprises frequency modulating said oscillator by picture signals and recurrent synchronizing pulses, deriving from the resulting frequency modulated signal a control voltage which changes in response to changes in the frequency representative of the peaks of said pulses, and utilizing said control voltage to make a predetermined frequency always representative of said peaks.

'7. A frequency modulation transmitter comprising a carrier wave oscillator, means for producing a modulating signal containing periodically recurring pulses which go to a predetermined voltage level, means for varying the frequency of said oscillator in accordance with the instantaneous amplitude of said modulating signal, and means for limiting said frequency variation so that it does not exceed a certain frequency limit for the lower frequency components of said pulses, said last means comprising a frequency discriminating circuit, means for applying to said frequency discriminating circuit a voltage having a frequency which varies in accordance with any changes in the frequency representative of said pulse peaks to obtain a control voltage which varies in amplitude in accordance with changes in said last mentioned frequency, and means for so controlling the frequency of said oscillator liu accordance with said control voltage that said' frequency limit is not substantially exceeded.

8. A frequency modulation transmitter comprising a carrier wave oscillator. means for varying the frequency of said oscillator in accordance with the instantaneous amplitude of a modulating signal, and means for limiting said frequency variation so that it does not exceed a certain frequency limit for at least certain frequency components of said signal, said last means comprising a stable oscillator which produces a voltage at a guard frequency located just beyond said frequency limit, means for mixing the voltages having said limit frequency and said guard frequency to produce a voltage at the diffrence frequency, a frequency discriminating circuit to which said last voltage is vapplied whereby there is-obtained a control voltage which varies in amplitude in accordance with changes in said beat frequency, and means for so controlling the'frequency of said oscillator in accordance with said control voltage that said frequency limit for at least said components is not substantially exceeded.

9. A frequency modulation transmitter comprising a carrier Wave oscillator, means including a reactance tube for varying the frequency of said oscillator in accordance with the instantaneous amplitude of a modulating signal, an amplifier through which the modulating signal is supplied to said reactance tube, and means for limiting said frequency variation so that it does n'ot exceed a certain frequency limit for at least certain frequency components of said signal, said last means comprising a stable oscillator which produces a voltage at a guard frequency located just beyond said frequency limit, means for mixing the voltages having said limit frequency and said guard frequency to produce a voltage at the difference frequency, a frequency discriminating circuit to `which said last voltage is applied whereby there is obtained a control voltage which variesy in amplitude in accordance with changes in said beat frequency, and means for controlling the gain of said amplifier in accordance with said control voltage so that said frequency limit is vnot substantially exceeded.

' 10. A frequency modulation transmitter comprising a carrier wave oscillator, an amplifier for amplifying a modulating signal, means for varying the frequency of said oscillator in accordance with the instantaneous amplitude of said modulating' signal, and means for limiting said frequency variation so that it does not exceed a certain frequency limitfor at least certain frequency components of said signal, said last means comprising a frequency discriminating circuit, means for applying to said frequency discriminating circuit a voltage having a frequency which varies in accordance with any changes in the frequency representative of said certain frequency components to obtain a control voltage which varies in amplitude in accordance with changes in said last-mentioned frequency, and means for controlling the gain of said amplifier in accordance with said control voltage so that said frequency limit is not substantially exceeded.v

11. A radio transmitter comprising a carrier wave oscillator, means for frequency modulatv ing said oscillator in accordance with a modulating signal which includes control pulses which go to a predetermined voltage level whereby the peaks of said pulses are represented by a certain maximum deviation of said oscillator frequency,

control means including a-stable oscillator for causing the peaks oi' said pulses always to swing the output of said oscillator to a predetermined frequency, and means for effectively connecting said control means to said oscillator during and only during the occurrence of said control pulses.

l2: A radio transmitter comprising a carrier wave oscillator, means including a reactance for frequency modulating said oscillator in accordance with a signal comprising a modulating signal and -recurring control' pulses whereby'thev peaks of said pulses are represented by a certain maximum deviation of said oscillator frequency, and means for causing the peaks of said pulses always to swing the output of said oscillator to a predetermined frequency, means including a trol bias which changes in response to a change in the frequency representative of said peaks,

`vkeying means for making said stable oscillator eiecti've for producing said bias during and only during the occurrence of said control pulses, and means for applying said control bias to said reactance tube. t

13. A picture transmitter comprising a carrier wave oscillator, means for frequency modulating said oscillator in accordance with a signal comprising picture signals and yrecurring synchronizing pulses which go to a fixed voltage level whereby the peaks of said'pulses are represented by a certain maximum deviation of said oscillator frequency, limiter means through which frequency modulated signal is passed for removing substantially all the signal except that representative of` said pulses, and control means -nal is supplied to said reactance tube, and means for limiting said frequency variation so that it stable oscillator for producing a frequency 4condoes not exceed a certain frequency limit for at least certain frequency components of said signal, said last means comprising a stable oscil- Y lator which produces a voltage at a guard frequency located just beyond said frequency limit, limiter means through which frequency modulated signal is passed for removing substantially all the signal except that representative of low frequency large amplitude components, means for mixing the voltages having said limited signal and said guard frequency signal to produce a voltage at the dierence frequency, a frequency discriminating circuit to which said last voltage is applied whereby there is obtained a control voltage which varies in amplitude in accordance with changes in said beat frequency,

and means for controlling the gain of said amplifler in accordance with said, control voltage so that said frequencyfllmit is not substantially exceeded.

THOMAS L.GO'ITIER. 

